3D Multi-PCB Design Achieves Greater Density in FSBB Converters
This article will be focused on a multi-PCB structure specifically designed for the four-switch buck boost (FSBB) converter.
The trend of current electronic applications, particularly those based on high-power devices, is to reach ever-smaller size with an ever-greater density of components. Thanks to the introduction of superjunction devices and wide-bandgap materials (such as gallium nitride), a higher switching frequency has been rapidly achieved, thus reducing the volume of passive devices.
This is true with the exception of the planar inductor. Because its loss is inversely proportional to its volume, its volume in high-density power supplies is increasing, wasting precious area on the PCB. By adopting a 3D multi-PCB structure, the volume of the power supply can be reduced until it reaches the size of the planar magnetic core. Multi-PCB layout can also improve heat dissipation and reduce parasitic inductance.
This article will be focused on a multi-PCB structure specifically designed for the four-switch buck boost (FSBB) converter, in which both power and control PCBs are located on the surface of the planar inductor, forming a 3D structure. This layout reduces overall volume, improves power density, and optimizes the power loop, thus reducing the stray inductance and the voltage overshoot. (The original article can be found here.)
The conventional FSBB layout
Due to its simplicity and the excellent performance of both voltage step-up and step-down, the FSBB converter is widely used in battery-charging applications. The circuit diagram, shown in Figure 1, is formed by a buck stage, a boost stage, and a shared inductor.
By adopting the quadrangle control method, the ZVS condition of all power switches can be achieved, allowing a switching frequency up to 1 MHz. As shown in Figure 2, the inductor current is shaped into a quadrangle, where the four corner currents are used to achieve zero-voltage switching (ZVS) (S2, S4, S1, and S3, respectively). These waveforms are obtained by regulating the phase shift between the buck and the boost stage. The ZVS technique allows the converter to activate the “soft switching” control, eliminating the switching losses that are normally produced during traditional PWM and synchronization.
The conventional single-PCB layout of an FSBB converter is shown in Figure 3. It includes four GaN power transistors, two gate drivers, a planar inductor, an auxiliary power supply, and an MCU, all distributed on the same plane.
The multi-PCB FSBB layout
To optimize the area occupancy, the components can be arranged in different PCBs, connected by means of a flexible printed circuit, as shown in Figure 4.
Figure 4: The 3D multi-PCB solution
A comparison between the 3D view of the traditional planar layout and the proposed multi-PCB layout is shown in Figure 5. The second solution drastically reduces the overall size, increasing the power density by 3.8×, from 90 W/in.3 to 432 W/in.3. Power devices are placed on the outermost side to improve heat dissipation, while an insulated shielding layer is inserted between the control board and the planar inductor to prevent the leakage magnetic flux from interfering with the logic controller (MCU). A ground connection between power and control boards is made up with copper foil or directly by welding together the two parts.
An additional benefit arising from the adoption of the multi-PCB design is a significant reduction of the overall length of the power loop, from 249 mm to 82 mm (see Figure 6). Besides, with the novel solution, the input and output terminals are closer to the power switches. A shorter power loop means a reduction of the parasitic inductance and, consequently, of the voltage overshoot caused by high-frequency switching.
Experimental results
To validate the design and assess the relevant electrical performance, a prototype for the proposed 3D multi-PCB FSBB converter has been set up. The converter prototype, shown in Figure 7, features an extremely compact footprint (26.7 × 25.0 × 15.9 mm), reaches a power density of 432 W/in.3, and provides the following technical specifications:
- Input voltage (VIN) from about 36 V to 72 V
- Output voltage (VO) = 48 V
- Maximum output current (IO) = 6 A
- Switching frequency (fS) = 1 MHz
The execution of accurate experimental tests made it possible to obtain the trend of the efficiency curves for different values of the input voltage and different load conditions, as shown by the graphs in Figure 8. As shown in the picture, the measured peak efficiency is 98.1% at 48-V input 3-A load, while the full-load efficiency at the nominal input voltage is 97.0%.
A comparison between the voltage overshot of the planar single-PCB solution and the proposed 3D multi-PCB solution is shown in Figure 9. As highlighted by the red dotted areas, the proposed solution can reduce the maximum value of the voltage overshoot from 37.5 V to 4.2 V. Due to its shorter power loop and its reduced voltage overshoot, the proposed multi-PCB solution is more suitable for high-frequency switching.
Conclusion
This article has proposed a novel 3D multi-PCB structure for a high-power–density FSBB converter. With respect to conventional planar single-PCB solutions, the two half bridges of the FSBB converter are placed on two PCBs, which are arranged symmetrically on the left and right sides of the planar inductor. The control board with the MCU is instead placed on the bottom layer of the planar inductor. Both power and control stage PCBs are wrapped around the planar inductor to form a 3D structure, which resembles the form of a cuboid.
The 3D structure not only drastically reduces the volume of the whole system, eliminating the wasted area, but it also greatly improves the power density. Additional benefits arising from the adoption of the 3D structure are a shorter power loop and small parasitic inductance, which, in turn, result in smaller voltage overshoot during high-frequency switching. The experimental results obtained with a 1-MHz 280-W FSBB prototype have demonstrated the validity of the design, with a peak efficiency of 98.1% achieved at 48-V input 3-A load and a power density increase (with respect to the conventional single-PCB design) from 90 W/in.3 to 432 W/in.3.
